1. Field of the Invention
The present invention relates to an audio amplifier, and more particularly, to a quaternary/ternary modulation selecting circuit of an audio amplifier and an associated method.
2. Description of the Prior Art
In audio applications, an analog amplifier is one of the most important parts in an audio system. Efficiency is a critical issue; a class D power amplifier is the most popular kind used in audio systems due to its higher efficiency compared to other kinds of amplifier. Because the output waveform of the class D power amplifier is a modulation signal between two voltage levels (i.e. supply voltage and ground) rather than a common linear waveform, ideally no current will pass through when the transistors of the output stage are non-conductive. Currently, a common modulation method applied in the class D power amplifier is Pulse Width Modulation (PWM), wherein quaternary modulation has the advantages of better total harmonic distortion (THD) and lower noise floor and is better for operating under small power conditions; ternary modulation, however, has the advantages of better efficiency and better Electro Magnetic Interference (EMI) performance and is better for operating under large power conditions. A quaternary/ternary modulation selecting circuit which can combine the above advantages is therefore a popular architecture in prior art class D amplifiers. There is an inrush current issue when the mode of the selecting circuit changes to the quaternary modulation from the ternary modulation, and when the selecting circuit starts up at power on, both of which might cause damage to the device. Consequently, how to achieve over-current protection in a quaternary/ternary modulation selecting circuit is an important issue in this field.
FIG. 1 is a diagram illustrating the over-current occurring in a prior art class D power amplifier. As shown in FIG. 1, the inrush current occurs in the first two pulses of the quaternary signal immediately after the mode of the quaternary/ternary modulation selecting circuit changes to the quaternary modulation from the ternary modulation and in the first two pulses of the quaternary signal immediately after the amplifier starts up. In addition, the current increases when the quaternary signal is high and goes down when the quaternary signal is low. Therefore, if the duty cycles of the first few signals are too high, the inrush current becomes much stronger.